The invention relates to an Inductively Coupled Plasma Mass Spectrometry Device (referred to hereinafter as an ICP-MS), and in particular relates to a device of this type which makes it possible to perform element analysis under a condition where the ionization rate and the interfering ion level are optimized by controlling a plasma potential of an Inductively Coupled Plasma (referred to hereinafter as an ICP).
Relevant prior art is disclosed, for example, in "The Basis and Application for the ICP Emission Analysis" by Haraguchi, published by the Koudan-sha Scientific, pages 13 to 19 and 99 to 104. FIG. 2 shows a part of the prior art which will be compared with the present invention. The device shown in FIG. 2 includes a plasma torch 1, a high-frequency coil 2, a gas control unit 3, a sprayer 4 for producing a fine spray, a sample solution 5, a spray chamber 6, a sampling orifice 7, an analysis tube 8, and an ICP 9. The plasma torch 1 is supplied, from the gas control unit 3, with a gas (for example, argon) which forms the plasma. The sample solution 5 is mixed in sprayer 4 with the gas from the gas control unit 3, and is sprayed in the form of a mist into spray chamber 6. The droplets in the mist are classified in spray chamber 6 and droplets having a diameter equal to or less than a predetermined diameter are transferred to plasma torch 1.
High-frequency coil 2 is supplied with high-frequency electric power at 27.12 MHz (or 40 MHz) by a high-frequency power source and a matching circuit (both not shown). IPC 9 is maintained by being inductively coupled with an alternating magnetic field generated by the high-frequency electric power in coil 2.
One end of IPC 9 is arranged with the analysis tube 8 which is exhausted by a vacuum pump (not shown) having a hole of about 1 mm in diameter as a sampling orifice 7 at the tip of it. The sample solution in the form of a mist is ionized within ICP 9 and introduced into the analysis tube 8. In the analysis tube 8, the ions are mass-separated by a mass filter (for example, a quadruple mass spectrometric device, not shown), and detected by a detector (for example, a channel-tron, not shown). Infinitesimal impurity elements in the sample solution are subjected to identification and determination based on mass and intensity of the ions thus detected.
In respect to a method of introducing the sample into the ICP there are various kinds of methods such as a method of heat introduction by electrical heat and a method of supersonic atomization and the like as disclosed in "The Basis and Application for the ICP Emission Analysis" by Haraguchi, published by the Koudan-sha Scientific, at pages 61 to 72, in addition to a method of sample spraying using the sprayer as shown in FIG. 2.
In the prior art there has not yet been a means for controlling ICP plasma potentials, accordingly ICP plasma potentials have varied depending on the status of the introduced samples. ICP plasma potentials will also vary depending on the grounding position of the high-frequency coils. If the ICP has a higher plasma potential, divalent ions of the impurity element in the sample solution to be detected or constituent ions of the sampling orifice are produces as interfering ions. If the ICP has too low a plasma potential, there exist elements (elements having higher ionization potentials such as iodine, bromine, and the like) in which detecting sensitivity is lowered due to a reduction of ionization rate. Further, the plasma potential of the ICP also affects the generation of oxide ions of the impurity element to be detected and interfering ions (ArO interfering with iron, ArAr interfering with selenium, and the like) caused by solvent of the sample or the constituent gas of the plasma. In the prior art, sensitivity to the interfering ions could not be controlled because the potentials of the ICP could not be controlled.